Packed reactors are commonly used in the chemical, pharmaceutical, waste treatment, and fermentation industries for a variety of processes. In general, packing provides an increase in surface area inside the reactor, based on the surface area to volume ratio of the particular packing selected. Generally, the surface area of the packing provides a surface onto which reaction promoters such as microbes may attach in biological reactors, or onto which reaction promoters such as chemical catalysts may be attached in chemical reactors. Since the reaction process whether biological or chemical is generally dependent upon reactant contact and time of contact with the reaction promoter, providing larger surface areas with the reaction promoter within a reactor volume may facilitate reactor size reduction leading to cost savings. Accordingly, the larger the surface area to volume ratio, the smaller volume of reactor is necessary for a particular process.
Commercially available packing varies widely in shape and may include elements to increase the surface area. For example, a cylindrical packing material may have an axial through bore, to provide both inner and outer surfaces to increase the surface area to volume ratio. In addition, the cylindrical packing may include inner and/or outer ribs or other structure to further increase effective surface area.
Referring for example more specifically to biological reactors used in waste treatment, the pollutants serve as a food source, generally as a source of carbon and/or nitrogen, for microorganisms on the reactor packing. Bacterial metabolism converts the pollutants to metabolites generally with a simple chemical structure, sometimes degrading the pollutants completely to carbon dioxide and water in an aerobic process, or to methane in an anaerobic process. But in any event, the metabolites usually have no adverse environmental effects.
Various bioremediation processes are known. For example, U.S. Pat. No. 4,634,672 describes biologically active compositions for purifying waste water and air which comprises a polyurethane hydrogel containing surface active coal having a specific surface according to BET of above 50 m.sup.2/g, a polymer having cationic groups and cells having enzymatic activity and being capable of growth. U.S. Pat. No. 4,681,852 describes a process for biological purification of waste water and/or air by contacting the water or air with the biologically active composition of U.S. Pat. No. 4,634,672. The experimental examples of these patents indicate that the process is not effective for reducing contaminant concentrations in the effluent strain to less than 44 parts per million (ppm). This is not acceptable since the Environmental Protection Agency (EPA) in some instances has mandated that concentration for some contaminants (such as phenol) in the effluent stream must be as low as 20 parts-per-billion (ppb). (See Environmental Protection Agency 40 CFR Parts 414 and 416. Organic Chemicals and Plastics and Synthetic Fibers Category Effluent Limitations Guidelines, Pretreatment Standards, and New Source Performance Standards. Federal Register, Vol. 52, No. 214, Thursday, Nov. 5, 1989. Rules & Regulations, 42522.
Both U.S. Pat. Nos. 3,904,518 and 4,069,148 describe the addition of activated carbon or Fuller's earth to a suspension of biologically active solids (activated sludge) in waste water as an aid in phenol removal. The absorbents presumably act by preventing pollutants toxic to the bacteria from interfering with bacterial metabolic activity. The patentees' approach has matured into the so-called PACT process which has gained commercial acceptance despite its requisites of a long residence time, copious sludge formation with attendant sludge disposal problems, and the need to regenerate and replace spent carbon.
Rehm and coworkers have further refined the use of activated carbon in the aerobic oxidation of phenolic materials by using microorganisms immobilized on granular carbon as a porous biomass support system. Utilizing the propensity of microorganisms to grow on and remain attached to a surface, Rehm used a granular activated carbon support of high surface area (1300 m.sup.2/g) to which cells attached within its macropores and on its surface, as a porous biomass support system in a loop reactor for phenol removal. H. M. Ehrhardt and H. J. Rehm, Appl. Microbiol. Biotechnol., 21, 32-6 (1985). The resulting “immobilized” cells exhibited phenol tolerance up to a level in the feed of about 15 g/L, whereas free cells showed a tolerance not more than 1.5 g/L. It was postulated that the activated carbon operated like a “buffer and depot” in protecting the immobilized microorganisms by absorbing toxic phenol concentrations and setting low quantities of the absorbed phenol free for gradual biodegradation. This work was somewhat refined using a mixed culture immobilized on activated carbon [A. Morsen and H. J. Rehm, Appl. Microbiol. Biotechnol., 26, 283-8 (1987)] where the investigators noted that a considerable amount of microorganisms had “grown out” into the aqueous medium, i.e., there was substantial sludge formation in their system.
Suidan and coworkers have done considerable research on the analogous anaerobic degradation of phenol using a packed bed of microorganisms attached to granular carbon [Y. T. Wang, M. T. Suidan and B. E. Rittman, Journal Water Pollut. Control Fed., 58 227-33 (1986)]. For example, using granular activated carbon of 16.times.20 mesh as a support medium for microorganisms in an expanded bed configuration, and with feed containing from 358-1432 mg phenol/L, effluent phenol levels of about 0.06 mg/L (60 ppb) were obtained at a hydraulic residence time (HRT) of about 24 hours. Somewhat later, a berl-saddle-packed bed and expanded bed granular activated carbon anaerobic reactor in series were used to show a high conversion of COD to methane, virtually all of which occurred in the expanded bed reactor; P. Fox, M. T. Suidan, and J. T. Pfeffer, ibid., 60, 86-92, 1988. The refractory nature of ortho-cresols and meta-cresols toward degradation also was noted.
Givens and Sack, 42nd Purdue University Industrial Waste Conference Proceedings, pp. 93-102 (1987), performed an extensive evaluation of carbon impregnated polyurethane foam as a microbial support system for the aerobic removal of pollutants, including phenol. Porous polyurethane foam internally impregnated with activated carbon and having microorganisms attached externally was used in an activated sludge reactor, analogous to the Captor and Linpor processes which differ only in the absence of foam-entrapped carbon. The process was attended by substantial sludge formation and without any beneficial effect of carbon.
The Captor process itself utilizes porous polyurethane foam pads to provide a large external surface for microbial growth in an aeration tank for biological waste water treatment. The work described above is the Captor process modified by the presence of carbon entrapped within the foam. A two-year pilot plant evaluation of the Captor process itself showed substantial sludge formation with significantly lower microbial density than had been claimed. J. A. Heidman, R. C. Brenner and H. J. Shah, J. of Environmental Engineering, 114, 1077-96 (1988). A point to be noted, as will be revisited below, is that the Captor process is essentially an aerated sludge reactor where the pads of sludge are retained in an aeration tank by screens in the effluent line. Excess sludge has to be continually removed by removing a portion of the pad via a conveyor and passing the pads through pressure rollers to squeeze out the solids.
H. Bettmann and H. J. Rehm, Appl. Microbial. Biotechnol., 22, 389-393 (1985) have employed a fluidized bed bioreactor for the successful continuous aerobic degradation of phenol at a hydraulic residence time of about 15 hours using Pseudomonas putida entrapped in a polyacrylamide-hydrazide gel. The use of microorganisms entrapped within polyurethane foams in aerobic oxidation of phenol in shake flasks also has been reported; A. M. Anselmo et al., Biotechnology B.L., 7, 889-894 (1985).
Known bioremediation processes suffer from a number of inherent disadvantages. For example, a major result of increased use of such processes is an ever increasing quantity of sludge, which presents a serious disposal problem because of increasingly restrictive policies on dumping or spreading untreated sludge on land and at sea. G. Michael Alsop and Richard A. Conroy, “Improved Thermal Sludge Conditioning by Treatment With Acids and Bases”, Journal WPCF, Vol. 54, No. 2 (1982), T. Calcutt and R. Frost, “Sludge Processing—Chances for Tomorrow”, Journal of the Institute of Water Pollution Control, Vol. 86, No. 2 (1987) and “The Municipal Waste Landfill Crisis and A Response of New Technology”, Prepared by United States Building Corporation, P.O. Box 49704, Los Angles, Calif. 90049 (Nov. 22, 1988). The cost of sludge disposal today may be several times greater than the sum of other operating costs of waste water treatment.
Use of anaerobic sewage treatment systems has been offered as a solution to the sludge problem. William J. Jewell “Anaerobic Sewage Treatment”, Environ. Sci. Technol., Vol. 21, No. 1 (1987). The largest difference between aerobic and anaerobic systems is in cellular yield. More than half of the substrate removal by aerobic systems can yield new microbial mass or sludge, the yield under anaerobic conditions is usually less that 15% of the organic substances removed. However, anaerobic systems are limited in the number of substrate that they can degrade or metabolize such as non-substituted aromatics (See N. S. Battersby & V. Wilson. “Survey of the anaerobic biodegradation Potential of Organic Chemicals in Digesting Sludge.” Applied & Environmental Microbiology, 55(2):p. 433-439, February 1989. This is a significant disadvantage in that most industrial processes, such as coke production and coal tar processing, normally produces non-substituted aromatics as by-products (See J. M. Thomas, M. D. Lee, M. J. Scott and C. H. Ward, “Microbial Ecology of the Subsurface at an Abandoned Creosote Waste Site.” Journal of Industrial Microbiology, Vol. 4, p. 109-120, 1989.
Another disadvantage inherent in some known bioremediation processes is that these processes do not reduce the levels of organic pollutants to reasonable levels [preferable less than about 0.1 parts per million (ppm)] at reasonable residence times (preferably less than about 24 hours). For example, in the process of U.S. Pat. Nos. 4,681,851 and 4,634,672 (see the specific examples), the concentration of phenol contaminants was not reduced below about 44 ppm.
U.S. Pat. No. 2,812,031 relates to the extraction of phenolic materials from aqueous solutions by means of polyurethane foam in the presence of hydrophilic fibers. The patent states that while polyurethane foams are relatively hydrophobic which can interfere with the interfacial contact which is necessary to permit adsorption, the problem is overcome through the use of hydrophilic fibers which enable the materials to come into close and in intimate contact with the surfaces of the polyurethane to facilitate wetting thereof.
U.S. Pat. No. 3,617,531 relates to a method for the selective adsorption of phenol from hydrocarbon solutions. In this method, the solution is contacted with polyurethane foam.
U.S. Pat. No. 4,983,299 and PCT WO 90/11970 describe fixed bed reactors for the bioremediation or organic contaminants where the reactor contains a biomass formed from particulates having a substrate such as polyurethane foam having anaerobic microbes and an absorbent for the pollutant on, in or on all in said substrate.
U.S. Pat. No. 4,165,281 discloses a method and a unit for wastewater treatment with microorganisms, in which at least one non-woven fibrous mat having a three-dimensional network structure is disposed as a supporting media in an aeration tank, microorganisms are retained on the surface of and in the interstices of the non-woven fibrous mat, and organic polluting matter in the wastewater is oxidatively decomposed by the microorganisms in the presence of oxygen.
U.S. Pat. No. 4,820,415 discloses a process for the biological treatment of an aqueous waste containing liquid by the removal of organic matter by microorganisms wherein a carrier material for said microorganisms is added to said liquid and wherein said carrier material comprises a filler-containing, hydrophilic, open-celled polymer in the form of separate individual particulates, the improvement wherein said polymer particulates, when saturated with water and charged with at least 70 volume % of biomass formed in the course of the process, have an average density of slightly below the density of said liquid and thereby are suspended in the upper two-thirds of said liquid.
U.S. Pat. No. 4,469,600 describes the biological purification of wastewater in a reactor in the presence of open-pore and compressible carrier material for biomass, the carrier material, prior to its use in the reactor, is loaded with bacteria, finely divided, inorganic and/or organic compounds, selected for wastewater purification, and is then either stored or used in the process, the loaded carrier being especially useful for decreasing the start-up time of a wastewater treatment plant.
U.S. Pat. No. 4,576,718 relates to the use of non-floating, non-abrasive, highly-filled polyurethane (urea) compositions of high water-absorbability, which during their production contain no cells capable of growth as carriers for biomass in the biological treatment of waste containing liquids. These carriers have a filler content of greater than 15% by weight and less than 95% by weight (based on the moisture-free). The fillers are selected from the group consisting of natural materials containing finely-divided fossil lignocellulose or the secondary products thereof (e.g., peat, lignite, mineral coal or coke), active carbon, finely-divided distillation residues, inorganic fillers, homogeneous or cellular plastics particulates (and more particularly polyurethane foam (waste) particulates) and mixtures thereof. The polyurethane (urea) is a hydrophilic and/or hydrophobic polyurethane (urea), and preferably contains cationic groups. These highly-filled, polyurethane (urea) carriers have a water-absorbability exceeding 33% by weight of water in the swollen carrier.
The prior art describes the superior properties of open cell polyurethane foam as a support matrix for biological active biomass but does not address the intrinsic problems associated with the use of this material in bioreactors. Mass transfer in fixed film bioreactors using polyurethane as a support matrix is limited by the structural integrity of the polyurethane foam to resist compression and thus by-passing of the fixed bed reactor by water and air as described in L. J. DeFilippi and F. Stephen Lupton, “Introduction to Microbial Degradation Of Aqueous Waste and its Application Using a Fixed-Film Reactor”, Chapter 2, p. 35-68, in “Biological Treatment of Hazardous Wastes”, ed. G. A. Lewandowski and L. J. DeFilippi, 1998. There have been a number of attempts to over come this inherent problem with polyurethane foam. One approach is to use polyurethane foam attached to rotating plates of a Rotating Biological Contactor (RBC) as described by Takahiro Suzuki, Satomi Yamaya and Masaru Ishida, “Treatment of Hydrocarbons in Air-Sparged Bioreactor and Rotating Biological Contactors”, The Association for Environmental Health and Sciences, Soil Sediment and Water, August, 2001. However, these authors show that RBC's are not as effective bioreactors as air-sparged (bubble column) bioreactors.
U.S. Pat. No. 5,217,616 relates to the use of polyurethane foam randomly mixed with hard plastic spacers to prevent compression of the polyurethane during air-sparging of the bed. Although the polyurethane bed is not compressed the mass transfer of oxygen from the air bubbles to the liquid phase is lower than if the liquid phase was aerated without the presence of the packing material.
Accordingly, it is desirable to develop a packing for a reactor that provides an advantageous surface area to volume ratio, while maintaining low pressure drop characteristics and avoiding excessive channeling of fluid through the reactor so that a distribution of residence times within the reactor is within a relatively narrow range. In addition, it is desirable to utilize materials, such as polyurethane foams, that have potentially high surface area to volume ratio due to pores within the foam that are accessible to provide reaction surfaces. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.